1. Field of the Invention
This invention relates to processes and catalysts that convert C1-C8 hydrocarbons to corresponding partially oxygenated compounds, such as methane to methanol, using heterogeneous catalysts.
2. Background of the Art
Activation and oxidation of lower alkanes (C1-C8) into useful oxygenates has long been an attractive and challenging research area. One reason for this interest is the fact that lower alkanes, especially methane (CH4) and ethane (C2H6), are predominant constituents of natural gas, which is currently both abundant and inexpensive. However, activation of lower alkanes often requires severe conditions using heterogeneous catalysts, for example, a temperature greater than 500 degrees Celsius (° C.) combined with increased pressure. Under these reaction conditions the valuable oxygenate products are, unfortunately, not stable, and the formation of carbon oxides, such as carbon monoxide (CO) and carbon dioxide (CO2), is usually observed.
In view of this problem, it is generally considered to be desirable to work at milder conditions, such that the formation of CO and/or CO2 is reduced or eliminated and the stability of the oxygenate products formed is enhanced. To enable these milder conditions, some researchers have explored activation of CH4 in the liquid phase, instead of in the gas phase. For example, B. Michalkiewicz, et al., J. Catal. 215 (2003) 14, reports the oxidation of CH4 to organic oxygenates at 160° C. and a CH4 pressure of 3.5 megapascals (MPa), using metallic palladium dissolved in oleum. That reference claims that methanol is obtained by the transformation of the CH4 to methyl bisulfate and dimethyl sulphate, and the ester is subsequently hydrolyzed. Unfortunately, use of strong acidic media such as sulfuric acid involves corrosive, toxic reaction conditions and a large amount of waste. Other research using oleum is reported in L. Chen, et al., Energy and Fuels, 20 (2006) 915, wherein vanadium oxide (V2O5) in oleum is employed, at 180° C. and a CH4 pressure of 4.0 MPa.
Mild conditions are also used in E. D. Park, et al., Catal. Commun. 2 (2001) 187, and Appl. Catal. A 247 (2003) 269, wherein selective oxidation of CH4 is carried out using hydrogen peroxide generated in situ, using a palladium/carbon (Pd/C) and copper acetate (Cu(CH3COO)2) catalyst system, with trifluoroacetic acid (TFA) and trifluoroacetic anhydride (TFAA) as solvents. The Pd/C serves as an in situ generator of hydrogen peroxide (H2O2), while the Cu(CH3COO)2 serves as the oxidation catalyst. The reaction conditions disclosed include 80° C., 5 mL solvent and a total gas pressure of 47.64 standard atmospheres (atm) (4.83 MPa) (71.4 percent (%) CH4, 14.3% hydrogen (H2), 14.3% oxygen (O2)). This process, too, requires formation of an ester followed by subsequent hydrolysis, and thus is not direct.
A method involving direct conversion of CH4 to oxygenate products is disclosed in Qiang Yuan, et al., Adv. Synth. Catal. 349 (2007) 1199, wherein CH4 is oxidized in an aqueous medium using H2O2 and homogeneous transition metal chlorides as catalysts. The transition metal chlorides may include, for example, iron chloride (FeCl3), cobalt chloride (CoCl2), ruthenium chloride (RuCl3), rhodium chloride (RhCl3), palladium chloride (PdCl2), osmium chloride (OsCl3), iridium chloride (IrCl3), platinum hydrochloride (H2PtCl6), copper chloride (CuCl2), and gold hydrochloride (HAuCl4). Unfortunately, in this process recovery and reuse of the homogeneous catalyst is difficult at best.
Other researchers have also addressed the use of microstructured catalysts. For example, Raja, et al., Applied Catalysis A: General, 158 (1997) L7, discloses a process to oxidize CH4 to methanol using phthalocyanine complexes of iron (Fe) and copper (Cu) encapsulated in zeolites as catalysts, and a combination of oxygen (O2) gas and tert-butyl hydroperoxide, which is in aqueous solution, as oxidants. The process includes an autoclave reactor and a suitable solvent, such as acetonitrile, and is carried out with the tert-butyl hydroperoxide at 273 degrees Kelvin (K) (0° C.) and a reaction time of 12 hours (h). The products include methanol, formaldehyde, formic acid and CO2.
Shul'pin, et al., Tetrahedron Letters, 47 (2006) 3071, discloses a process for the oxidation of alkanes (including CH4, C2H6, propane, n-butane, hexane, heptane, octane and nonane) using H2O2 as the oxidant to form the corresponding alcohols and ketones. The process includes an autoclave reactor with, in the case of CH4, a pressure of 50 bar (5 MPa) and a reaction time of 24 h. The catalyst is a titanium-containing zeolite, “TS-1” (silicon (Si) to titanium (Ti) ratio is 20, Si/Ti=20), and methanol is the main product (1.1 micromole (μmol) of methanol produced after 24 h).
Finally, Sorokin, et al., Chem. Commun., (2008) 2562, discloses the oxidation of CH4 under mild conditions (25-60° C., 32 bar (3.2 MPa) CH4 pressure, 678 μmol H2O2, and a reaction time of 20 h) using a μ-nitrido diiron phthalocyanine complex in water as a homogeneous catalyst and, additionally, a silica-supported μ-nitrido diiron phthalocyanine complex. The products include methanol, formaldehyde and formic acid, with formic acid being primary.
While researchers have identified a number of operable processes, there is still a need to identify additional processes that are both environmentally benign and economically attractive, and that desirably do not require intermediate steps or products in order to produce the desired final oxygenate products from C1-C8 hydrocarbons.